Cheap “Disposable Cameras” are now rarely used but are still available in eBay. I've bought a new one in a Chinese market, for the modest price of 4.45 € (!) In addition, in a tour of several camera stores in my area, I was able to collect several used and disposable units, for free, but with electronic flash still perfectly working for my purpose.

My idea was to create a neon light source for the spectroscope calibration as shown here, in the method and in the result, which proved to be very interesting.

Operation of the circuit:

T1, Q1 and R 1 form the oscillator circuit that lift the voltage from 1.5 V. to about 300 V.

SW1 is the button on the front of the camera. Its continued pressure activates the charging circuit of the capacitor C1, through the diode D1, up to the maximum voltage, as indicated by lamp L1.

L1 may be a small neon bulb (a LED in some versions), powered through the resistors R2 and R3.

T2 is a small transformer that multiplies the voltage up to 2000 V, needed to trigger the flash, when closing the shutter button SW2.

Transformation of the circuit:

Caution! : The capacitor C1 probably still has the power remaining at high voltage. Therefore, before operating on the circuit is indispensable fully discharge it, shorting its terminals with a screwdriver.

After you have completely discharged the capacitor C1 shorting its terminals with a screwdriver, desolder using a soldering iron and remove it.

Desolder and remove the flash lamp and, possibly, T2.

The small lamp L1, wanting to use it as a power ON indicator, can be left in position . Alternatively, but only in the event of small neon bulb, (not in the LED alternative , of course), L1 could be already used as a small Neon calibration weak source, without needs of further changes to the circuit. In this case it would be advisable to remote L1, unsoldering and then connecting it again to the circuit via a small cable.

In place of the capacitor C1, a new calibration neon bulb must be connected, with a connection cable and a resistor in series, whose ideal value is a function of the absorption of the new bulb. Several tests have shown that resistors from 200 k ohms to 500 k Ohm satisfy most of the cases. The photo shows the first prototype, where this resistance is realized temporarily by a 200k potentiometer, in series with a fixed resistor of 50K.

It is advisable to short-circuit the SW1 terminals and insert a small switch on the positive pole of the power battery, 1.5 V AA size, or even AAA.